The housing is most likely molded from austenite, which is non-magnetic form of steel, it is the crystal lattice that forms when low carbon steel is heated to very high temperatures and cooled quickly, lower temperature produces ferrite, which is also paramagnetic.

When more carbon is present, or when steel is heated to red hot and quenched, the result is either martensite, or cementite cyrstal formation, these structures are all ferromagnetic (to varying degrees, depending on carbon content) while austenite is paramagnetic. Paramagnetic materials only hold a magnetic field when an external field is applied, and returns to non-magnetic when the external magnetic field is removed.

In other words, austenite "passes" magnetic fields without becoming permanently magnetized itself, unlike ferromagnetic materials, which become magnetized when exposed to a magnetic field.

Lastly, metal detectors don't emit rays, they create an oscillating magnetic field, and typically have a co-mounted coil antenna to receive any magnetic abnormalities below the surface, such as an item becoming magnetized, or having a small electric current induced in it from the detectors' transmitting coil, which creates a small magnetic field.

There is also a possibility of any metallic housing for a search coil acting as a "shorted turn", that is, reducing the effectiveness of the coil due to eddy-current effects. This is likely to become worse as the operating frequency increases, but may be an issue even at audio frequencies.

Apart from using low frequencies and accepting some impairment, part of the housing may actually be made out of non-conductive material. This may only need to be a small section, just enough so that less than a complete shorted "turn" is formed. This may not be obvious, for instance steel plates or tubes may be interrupted by an insulating joint made of plastic etc. which may not be seen on casual inspection.